A general decontamination treatment for radioactive cesium contaminated grains and, in particular, for soils is performed by two methods. A first method is a method primarily performed by a mechanical treatment. In a contaminated soil, a soil having a small grain diameter in which a most part of radioactive cesium is contained and a soil having a large grain diameter in which a small part of radioactive cesium is contained are classified (Patent Literature 1), or as a first step, a radioactive cesium contaminated soil is combusted in a combustion furnace, and a volume-reduced contaminated soil is then classified into a portion containing a large amount of radioactive cesium and a portion containing a small amount of radioactive cesium by a classification device (Patent Literature 2).
A second method is a method in which radioactive cesium is extracted from a radioactive cesium contaminated soil using a chemical agent solution. Extraction chemical agent solutions described in Patent Literature 3 are iron salts, such as ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, and polyferric sulfate, and chloride compounds, such as an ammonium salt and a potassium salt. This extraction liquid is further treated by cesium chloride, glycerin, or ethylene glycol monoethyl ether (EGME: cellosolve).
On the other hand, extraction chemical agent solutions described in Patent Literature 4 are an inorganic acid, an organic acid, and the like, this acidic solution is neutralized by an alkali agent and is further ion-exchanged in a washing step using washing water which contains ammonium sulfate, and a supernatant is separated from a deposited soil. This supernatant is processed so that radioactive substances are absorbed by an absorbent, such as mordenite or zeolite.
In the category 2 radioactive actinoid elements, such as uranium and plutonium, extraction chemical agent solutions according to Patent Literature 5 are sodium carbonate, oxalic acid, succinic acid, and EDTA (ethylenediaminetetraacetic acid functioning as a chelating agent), and in addition, when a sodium salt of the above extraction chemical agent is changed to a potassium salt thereof, the extraction efficiency is improved. Unlike the case of cesium, since uranium, plutonium, and the like are not likely to be dissolved in water, an oxidant, such as hydrogen peroxide, ozone, or potassium permanganate, is required to be added to the extraction chemical agent solution mentioned above.
The present inventor confirmed that after an improvement/purification test is performed on a radiocontaminated soil using paper sludge-derived sintered carbonized porous grains, radioactive 134Cs and 137Cs can be removed from the radiocontaminated soil, and the present inventor also disclosed in Patent Literature 6 that 30 Bq/kg, which is the total value of radioactive 134Cs and 137Cs contained in obtained white rice, is smaller than a Japanese reference value of 100 Bq/Kg.
In this case, the paper sludge-derived sintered carbonized porous grains are formed by sintering/carbonizing paper sludge discharged from paper manufacturing mills which use either waste paper or wood chip or both waste paper and wood chip, and have the following configuration.
(1) Paper sludge discharged from paper manufacturing mills which use either waste paper or wood chip or both waste paper and wood chip is processed by sintering/carbonization to form paper sludge-derived sintered carbonized porous grains which have a pH of not less than 8 and preferably not less than 10; an alkalinity equivalent value of 1.0 to 4.0 meq/g (as NaOH) and preferably 1.5 to 2.5 meq/g (as NaOH); a cation exchange capacity of 1.0 to 4.0 meq/100 g (as NH4) and preferably 1.5 to 3.0 meq/100 g (as NH4); an electric conductivity of 70 to 150 μS/cm; a sodium (Na) content of not less than 0.0003%; a potassium (K) content of not less than 0.0003%; an organic content of less than 25%; and an inorganic content of not less than 75%, and the paper sludge-derived sintered carbonized porous grains thus obtained are dispersed on or mixed with radiocontaminated soil to remove radioactive substances therefrom.
(2) In the manufacturing process of the said paper sludge-derived sintered carbonized porous grains, the impregnation of the paper sludge with either potassium iodide (KI) alone or ethylenediaminetetraacetic acid (EDTA) alone or a combination of KI and EDTA was not incorporated.
(3) The radiocontaminated soil contains radioactive 134Cs and 137Cs at a total dosage of not less than 800 Bq/kg.
(4) The dosage of the said paper sludge-derived sintered carbonized porous grains spread on or mixed with the radiocontaminated soil is 0.1 to 6 kg/m2 (0.5 to 50 kg/m3) (0.1 to 6 percent by weight of dry soil) and preferably 1.0 to 3.5 kg/m2 (8 to 30 kg/m3) (0.9 to 3.3 percent by weight of dry soil).
(5) The paper sludge has a moisture content of 50% to 85%, and after being pelletized and dried, this paper sludge is pyrolyzed in a reducing carbonization sintering furnace at a temperature of 500° C. to 1,300° C., preferably 700° C. to 1,200° C. Furthermore, carbonization is preferably carried out at 800° C. to 1,100° C.
(6) The said paper sludge-derived sintered carbonized porous grains contain, on oven-dry weight basis, 15% to 25% of combustibles (including carbon), 0.5% to 3.0% of TiO2, 0.0001% to 0.0005% of Na2O, 0.0001% to 0.0005% of K2O, 15% to 35% of SiO2, 8% to 20% of Al2O3, 5% to 15% of Fe2O3, 15% to 30% of CaO, 1% to 8% of MgO, and a balance of 0.5% to 3.0% (including impurities), the total of these being 100%; and has a water absorption rate of 100% to 160% in accordance with JIS C2141, a specific surface area of 80 to 150 m2/g in accordance with the BET adsorption method, and an interconnected cell structure.
(7) The said paper sludge-derived sintered carbonized porous grains are to have a porosity volume rate of not less than 70%, a porosity volume of not less than 1,000 mm3/g, an average pore radius of 20 to 60 μm, and pores with radius of not less than 1 μm constitute not less than 70% of the total porosity volume, and are a mixture of various forms such as spherical, oval, or cylindrical or the like forms with each having an axis length of 1 to 10 mm, and a black color.